This invention relates to hydrogen generation, and more particularly, to a system for the catalytic hydrolysis of sodium borohydride.
As is well known, fuel cells combine hydrogen and oxygen to generate electricity. In some applications, it is necessary to precisely and accurately meter hydrogen, on demand, to a fuel cell to control the electrical output. An example is a miniaturized PEM fuel cell.
It is also known that hydrogen can be evolved from reactive solutions such as by the catalytic hydrolysis of sodium borohydride, NaBH4. A prior art approach for generating hydrogen on demand for use in fuel cells utilized pumps to deliver an aqueous solution of sodium borohydride to a catalyst bed. There are, however, problems with this approach. A stoichiometric sodium borohydride aqueous solution has high viscosity that makes it difficult to pump in a precise manner. Sometimes nonreactive chemicals were added to lower the viscosity or a nonstoichiometric ratio was used to aid pumping. Alternatively, a strong base, such as sodium hydroxide, was added to the sodium borohydride solution in an effort to stabilize it. The resulting solution was then caustic making handling more difficult.
Another problem with the prior art approach is that the hydrolysis reaction generates a by-product, sodium metaborate, NaBO4, that must be removed. Sodium metaborate tends to pick up water and to gel when allowed to cool. Its presence hinders access to the catalyst and makes the water needed for the reaction less available. If this reaction product is allowed to cool and gel, it cannot be readily pumped away.
Another approach to evolving hydrogen from sodium borohydride is to heat the material to generate hydrogen as needed. A problem with this approach is high parasitic loss and another problem may be material compatibility issues.
The hydrogen generation system of the invention includes a source of sodium borohydride along with a source of water and a catalyst such as ruthenium. Structure is provided for contacting sodium borohydride, water and catalyst to release hydrogen and to move the reactants into and away from a reaction zone.